US Defense Secretary Ash Carter, center left, is greeted with a military honor guard as he arrives to meet Afghan President Ashraf Ghani at the Presidential Palace in Kabul, Afghanistan, on Feb. 21, 2015. Defense Secretary Carter was making his international debut Saturday with a visit to Afghanistan to see American troops and commanders, meet with Afghan leaders, and assess whether US withdrawal plans are too risky to Afghan security.

Donald Trump has emerged as perhaps the most isolationist presidential candidate in a half-century, with an “America First” platform that criticizes costly military interventions abroad and questions US commitments to treaty allies that don’t pay their way.

In a major foreign policy speech on Monday, he invoked the cold war in vowing “foreign policy realism” that prioritized the fight against Islamic terrorism. “If I become president, the era of nation-building will be brought to a quick and very swift end,” he said in Youngstown, Ohio.

The approach represents a sharp detour from the longstanding bipartisan belief in a US-led liberal international order as a source of national strength. He is by no means alone, however. Prominent intellectuals have also sought to persuade Americans to rethink their country’s role in the world.

But Trump’s foreign policy is riddled with so many contradictions that the isolationism he and they espouse has mostly been lost in the anti-Trump backlash. That has sparked great frustration on the part of proponents who see a moment ripe for change amid public weariness of foreign entanglements and nation-building, as well as the cost of deploying the world’s largest military.

“Because it’s Trump who’s selling it, it’s a joke, so you don’t have to take his points seriously,” argues Ian Bremmer, whose recent book, “Superpower: Three Choices for America’s Role in the World,” casts a skeptical eye over the benefits to the US of effectively policing the world and projecting its democratic values overseas.

Stephen Walt, a professor of international studies at Harvard University and prominent critic of US military interventions, wrote on Foreign Policy’s website last week that Trump was “just about the worst salesman for an alternative foreign policy that one could possible imagine.”

Take Russia, where foreign policy realists like Mr. Walt see room for better relations based on mutual interests and an appreciation of Russia's own strategic weaknesses. But Trump's admiration for President Vladimir Putin and his campaign manager's ties to an ousted pro-Russian dictator in Ukraine have muddied the waters for any potential reset in relations.

Irked by both the message and the messenger, dozens of Republican national-security heavyweights, including a former CIA director and Homeland Security secretaries, have criticized Trump as not fit to be commander-in-chief. Hillary Clinton, who has a more expansive view of military power projection and frames the US as the indispensable global force that leads by example, has garnered a number of endorsements from such Republicans in recent weeks.

So if Trump loses in November, does the genie of isolationism go back in the bottle? Not necessarily, say Mr. Bremmer and other analysts.

“Even if Trump’s challenge to the foreign policy establishment fails, those sorts of concerns are not going to go away,” says Andrew Bacevich, professor of international relations and history at Boston University, who has called for fewer US troops overseas and greater restraints on their deployment.

Election impact

Since campaigns rarely turn on foreign policy, it’s hard to gauge how far insurgent candidates like Trump and Bernie Sanders benefited from speaking out against US interventions in global trouble spots – or where they fit into the current political system, given the consensus among Democratic and Republican foreign-policy elites since the cold war ended.

Gordon Adams, a professor emeritus at American University’s School of International Service who was a foreign-policy advisor to the Sanders campaign, says voters are skeptical of open-ended military deployments. “But it’s not very coherent and not the primary motivator for their unhappiness with the American political system,” he says, pointing to Sanders’s laser focus on economic policy.

Trump’s foreign policy is similarly grounded in domestic priorities. His campaign website has no entries on foreign affairs, aside from a section entitled “Compelling Mexico to pay for the wall.”

Yet framing “America First” as isolationism is difficult, as Trump’s positions are fluid, say analysts. Monday’s speech is a case in point: It argued for a mobilization of US resources to fight global Islamic terrorism and derided the Obama administration for letting the Middle East fall apart.

“What we heard today is not isolationism. It’s global engagement, albeit with a very specific and narrowly defined purpose and that is to destroy his notion of what threatens us,” says Mr. Bacevich, a retired Army colonel who previously taught at West Point.

Trump also argued that the US should have kept Iraq’s oil after its invasion which he claims he didn’t support. “In the old days when we won a war, to the victor go the spoils,” he said.

Mr. Adams says it’s impossible to fit Trump’s foreign policy into a conceptual box because of his scattershot approach. “This is not a vision. It’s a series of contradictory bumper stickers.”

The origins of 'America First'

A century ago, President Woodrow Wilson brought the US into World War I, ending a long period of forbearance from foreign wars that was rooted in the 1823 Monroe Doctrine.

During the 1930s, some Americans looked askance at political turmoil in Europe and opposed calls by Democratic politicians to combat the rise of fascism. Among the most prominent isolationists was aviator and Nazi admirer Charles Lindbergh, whose movement was also called America First. Lindbergh’s cause failed and the US military played a decisive role in World War II and in the postwar reconstruction of Europe and Asia.

Bremmer inadvertently provided the nomenclature for Trump by referring to his policy before the primary season as America First. When The New York Times asked Trump about it in April, he said he liked the expression and repeated it on the campaign trail.

Bremmer favors an approach that he calls Independent America – retrenching in order to build at home and attract allies via soft power. At first glance, it resembles Trump’s America First, a vision of a unilateralist power that no longer polices the planet. But Bremmer argues that Trump’s hostility to refugees and support for torture undercuts a crucial corollary, the leading-by-example that makes the US a magnet for the world’s capital and talent.

“While Trump absolutely gets that the average American no longer wants to spend trillions of dollars on endless wars in Iraq and Afghanistan, he has no interest in holding America up as an example for other countries,” he says.